1. Field of the Invention
The invention relates to a method for installing a duct, a longitudinal member being present along the section over which the duct is to be installed, the present longitudinal element being made use of as a conductor for the duct to be installed, and the duct being propelled.
2. Description of the Prior Art
A method of installing a duct is known from U.S. Pat. No. 5,639,183. The known method is intended for installing—instead of an existing telecommunication cable lying in the ground, particularly a copper cable—a duct through which an optical cable may be conducted. In the event of said method, the existing cable is excavated at a suitable point, and cut through. The duct is slid over an end released in this manner. To install the duct in the soil surrounding the existing cable over a greater length, a liquid—e.g., water—is conducted through the duct under pressure; it washes away the ground around the existing cable, while at the same time the duct is pushed forward. In this manner, a duct, or a duct consisting of several coupled segments, may be installed underground over a great distance along an existing cable, without much excavation work being required for said purpose. After having been installed, the cable, if so desired, may be pulled out of the duct. One of the advantages of the known method is that curvy sections are easy to follow, without expensive positioning equipment.
It should be noted that, in the present application, the word “duct” signifies any tubular member, without restriction as to its application, although in the application, more specifically, applying the inclusion of telecommunication cables is dealt with. In this connection, it is not impossible that the duct has already been provided, in advance, with one or more cables.
The known method is particularly suitable for replacing the existing copper telecommunication cable with a cable duct in which an optical fibre cable may be installed for installing, e.g., fibre up to the user, i.e. up to the home or company. The optical fibre may be installed in the duct, e.g., by way of blowing or pulling, installing by way of blowing being preferred, since in this case the forces exercised are spread out over the entire length and damages as a result of excess tensile forces are prevented. Such a method for blowing in a cable is described in EP-A-0,292,037. A combination of blowing in and pulling is also possible by attaching, at the end of the optical fibre cable, a so-called shuttle.
Under certain circumstances, it is undesirable to cut through the existing cable in order to thus be capable of sliding the duct over it. Such is the case, e.g., if the cable must remain available for other purposes.